It is becoming increasingly difficult, both in terms of cost and site availability, to construct conventional open reservoirs for the storage of water. Such reservoirs typically require the construction of a dam across a river, thereby flooding vast expanses of land upstream of the dam while severely curtailing the flow of water downstream from the dam. In light of the increasing value of water and the complexities of the various water laws across different jurisdictions, it is becoming prohibitively difficult to form an open reservoir in this manner.
A further disadvantage of open reservoirs is the high degree of evaporative losses experienced by such reservoirs due to the relatively large air/water interface. Specifically, in arid climates (such as those found in the Western United States), open reservoirs are subject to extremely large evaporative losses. Indeed, such evaporative losses are typically greatest where water is needed most.
Underground porosity reservoirs, such as those described in U.S. Pat. No. 6,840,710 to Peters et al., titled UNDERGROUND ALLUVIAL WATER STORAGE RESERVOIR AND METHOD, have been posited as an alternative to open reservoirs. Underground porosity reservoirs include a volume of porous material, such as natural alluvium, bounded by substantially impermeable walls to create an underground vessel capable of storing water. Underground reservoirs are not subject to evaporation losses and can potentially be used without the loss of surface use of the site.
Methods of operating an underground porosity reservoir are described in co-pending U.S. patent application Ser. No. 10/704,347, titled METHOD OF OPERATING A WATER STORAGE RESERVOIR HAVING POROSITY STORAGE, filed Nov. 7, 2003, which is incorporated herein by reference. Following initial steps of building the substantially impermeable walls and pumping entrapped water back to the surrounding groundwater system, the porosity storage reservoir is typically filled to capacity and then emptied to determine the net storage capacity of the reservoir. Filling the reservoir to capacity typically produces water levels within the reservoir that are higher than would otherwise occur naturally within the alluvium. Depending on the amount of fine-grained materials existing between the sand and gravel particles, several filling cycles may be required to flush out these relatively fine materials and thereby increase the net capacity of the porosity reservoir.
In order to maximize the storage capacity of a porosity reservoir, it is necessary to fill the reservoir to its highest level. However, the dimensions of a porosity reservoir may be vast (e.g., hundreds of acres of surface area and thousands of acre-feet in volume), and thus the surface of the land encompassed by the substantially impermeable walls may be gently sloped as opposed to level. This tendency is only amplified due to the fact that porosity reservoirs are typically constructed within the alluvial soils of a river bed so that there is an “upstream” and “downstream” portion to the reservoir corresponding to the overall valley gradient. In one example, if a porosity reservoir has an average depth of 50 feet but the surface level of the reservoir drops 10 feet from the upstream to the downstream end of the reservoir, it can be presumed that approximately ten percent of the overall volume of the porosity reservoir is not used when the reservoir is filled to the maximum level of the “downstream” end of the reservoir (i.e., an average of 5 feet from the upstream to the downstream end divided by the 50-foot depth). That is, when the reservoir is filled to capacity in a static state so that there is no water flow through the reservoir, the water level will be no higher than the height of the lowest “top” elevation at the downstream end of the reservoir. The portion of the reservoir that extends above this maximum static water level is referred to herein as the “wedge” since it constitutes a generally wedge-shaped portion when viewed in a section extending from the upstream to the downstream portion of the porosity reservoir.
In order to maximize the amount of water that can be stored within the underground porosity reservoir, an improved system is needed to store water in the wedge portion of the reservoir (i.e., recover some or all of the ten percent loss described above). It is with respect to these and other background considerations, limitations and problems that the present invention has evolved.